The primary objective of this trial is: 1. To evaluate the long-term safety and the tolerability (adverse experiences) of oral ridogrel 2.5 mg or 5.0 mg given for 24 months to pediatric patients with ulcerative colitis. The secondary objectives are: 1. To evaluate the pharmacokinetics of a single oral dose of 2.5 or 5.0 mg ridogrel in pediatric patients age >8 to <18 with active U.C. 2. To evaluate/document the long-term efficacy of oral ridogrel on endoscopic and clinical activity scores in pediatric patients with ulcerative colitis. 3. If possible, to evaluate the impact of the disease state (active versus non-active) on the pharmacokinetics of ridogrel. In December 1998 the Mallinckrodt General Clinical Research Center was notified by the principal investigator that the sponsor, Pharmaceutical Research Associates, Inc., had been notified by Jansen Research Foundation that enrollment has been suspended for both SPID 0395 and SPID 0396. This decision has been made as a result of data received from the pivotal trials.